Erosion-corrosion resistant coating

ABSTRACT

A process for improving the erosion resistance and corrosion resistance of steel articles which comprises boronizing said articles and thereafter coating said boronized articles with chromium under conditions such that the chromium combines with the boron to produce an article bearing a coating containing a boride rich in chromium on the surface of said article.

United States Patent [72] Inventors Sanlord Baranow Woodhridge; RichardB. Andrews, Milford, both of Conn. [21 Appl. No. 796,566 [22] Filed Feb.4, I969 [45] Patented Nov. 23, 1971 [73] Assignee Avco CorporationStratford, Conn.

[ 54] EROSION-CORROSION RESISTANT COATING 117/71, 106, 107.2, D10. 10;29/1966, l9, l 66 Primary Examiner-Ralph S. Kendall Attorneys-Charles M.Hogan, Irwin P. Garfinkle and Lawrence 1. Field ABSTRACT: A process forimproving the erosion resistance and corrosion resistance of steelarticles which comprises boronizing said articles and thereafter coatingsaid boronized articles with chromium under conditions such that thechromium combines with the boron to produce an article bearing a coatingcontaining a boride rich in chromium on the surface of said article.

EROSION-CORROSION RESISTANT COATING This invention relates to articlesof steel, including alloyed steels such as stainless steel, providedwith a chromium-boron coating and to the process whereby such articlesare produced. I

Many steels are known which possess the strength necessary for use inservice under conditions where the steel is subjected to considerablestress. However, these alloys are often deficient in resistance to theerosion and/or corrosion encountered in their intended service.

One object of the present invention is to provide improved articles suchas compressor blades which when exposed to the severe environmentalconditions incidental to the operation of a gas turbine engine, exhibita high degree of erosion resistance and a high degree of corrosionresistance and a fatigue life not substantially lower than that of thearticle before the coatings of the present invention are providedthereon.

Gas turbines are presently used to power helicopters and land vehiclesadapted to be utilized on sandy beaches and in other erosiveenvironments. The present invention provides erosion protection for thecomponents of said turbines without sacrificing any substantial portionof the fatigue life of the steel from which the parts are fabricated.

As is well known, steels can be boronized by various techniquesincluding pack cementation and chemical vapor deposition to obtain outersurface layers which contain ironboron compounds. Further, it is knownthat such coatings when properly formed possess extremely high-erosionresistance and markedly inferior corrosion resistance. With somematerials, e.g., AM350 Stainless Steel (nominal composition: 16.5percent Cr, 4.5 percent Ni, 3 percent M0, 0.1 percent N, balance Fe),the fatigue life of the resulting boronized article may be diminished byas much as 50 percent, as compared with the fatigue life on the articlebefore boronizing.

in one effort to avoid this impairment of corrosion resistance it hasbeen proposed to chromize the base material prior to boronizing, asdescribed, e.g., in US. Pat. No. 3,029,162. The surface of the resultingarticle retains the higherosion resistance possessed by simple boronizedsurfaces, and possesses an improved corrosion resistance, as comparedwith such boronized articles, although the corrosion resistance of suchchromized, boronized articles is only moderate. The fatigue life of sucharticles is still only about 50 percent of that of the original,uncoated material.

When the chromized, boronized article is examined under a metallurgicalmicroscope, the microstructure of the coating is observed to be similarto that of a simple boron coating, except that the chromium-boroncoating is found to be richer in chromium than the simple boron coating.

It has now been foundthat the improvement in erosion resistanceresulting from a boron coating on the article can be retained and theimprovement in corrosion resistance resulting from a chromium-boroncoating can be increased by a simple, but fundamentally significant,change in the procedure described in US. Pat. No. 3,029,162,specifically by changing the order of deposition of the coatingconstituents. In the present invention, the boron is applied first andthen the chromium is deposited on the articles to be protected.

When an iron or steel article is boronized, the coating formed is anouter layer comprising FeB and an inner layer of Fe B. Continuedintrusion of boron produces increasingly thick layers of FeB and Fe B.

If, as in Samuels US. Pat. No. 3,029,162, the article is coated withchromium, silicon, titanium, etc., and then boronized, the resultingarticles are found to comprise one or more FeCr-B compounds, as thesurface coating. The

result is an improvement in the corrosion resistance, as com pared witha boronized, but not previously chromium-coated article.

Now, in the present invention, the boron is deposited first and then thechromium is deposited under such conditions that it is able to combinewith the alread deposited boron. As a result, the outer layer of thearticle IS a most entirely Cr B or Cr B and very little Fe appears to bepresent in the outermost layers of the coating. This high chromium,boride coating is remarkably corrosion resistant as well as erosionresistant. in addition, the fatigue life of blades coated according tothe present invention is at least percent that of uncoated blades and,therefore, the resulting articles are serviceable in many uses wherelow-fatigue life bars use of the prior art coated articles.

The following example will serve to further illustrate the benefitsconferred by the present invention.

Twenty AM350 stainless steel alloy compressor blades were placed in aretort containing boron powder and 0.5 weight percent potassiumfluoride. The retort was heated in an inert atmosphere for 1 hour atl,900 F. in which time an iron boride coating of 0.00l-inch nominalthickness was formed. The blades were then packed in a retort containingchromium powder and heated in a vacuum for 4 hours at l,950 F. Afterbeing permitted to cool to room temperature, the blades were removedfrom the retort. Then various tests, including sand erosion, corrosionand fatigue, were performed. The erosion rate of B-Cr coated blades wasfound to be about 20 times lower than that for uncoated blades. Noattack was noted in salt-spray corrosion tests. The fatigue life of theblades was affected negligibly.

Other steels and other stainless steel alloys may be used in place ofthe specific alloy of the foregoing example and the boron and chromiummay be deposited on the articles by any other known methods, providedthat the order of deposition is as specified and that the boron-coatedarticle is exposed to an elevated temperature for a sufficient timeinterval for the Cr to combine with the B already present.

It will also be understood that unless there is sufficient boron to formthe desired high-chromium boride, the full benefits of the inventionwill not be realized. Hence the boronizing step should be carried out toform a boron-containing layer which is at least about 0.000 1 -inchthick.

Similarly, the amount of chromium applied to the boronized articles mustsufiice to form the intended high-chromium boride-coating on the finalarticle. Deposition may be by any known method, provided it does notremove or adversely affect the boron deposited on the articles.

Having now described a preferred embodiment of the invention, it is notintended that it be limited, except as may be required by the appendedclaims.

We claim:

1. A process for improving the erosion resistance and corrosionresistance of steel articles which comprises:

boronizing said articles. and thereafter chromizing said boronizedarticles to produce articles bearing a highchromium boride-containingcoating on the surface of said articles.

2. The process of claim 1 wherein the boron-containing coating firstlaid down is at least approximately 0.000l-inch thick.

3. The process of claim 1 wherein the chromium is diffused into theearlier deposited boron by subjecting the boronized articles totemperatures between about 1,800" F. and 2,l00 F. for between about 1and 16 hours.

4. An erosion resistant and corrosion resistant article produced by theprocess of claim 1.

2. The process of claim 1 wherein the boron-containing coating firstlaid down is at least approximately 0.0001-inches thick.
 3. The processof claim 1 wherein the chromium is diffused into the earlier depositedboron by subjecting the boronized articles to temperatures between about1,800* F. and 2,100* F. for between about 1 and 16 hours.
 4. An erosionresistant and corrosion resistant article produced by the process ofclaim 1.